Almost all commercial electronic integrated circuits rely upon active areas of singly crystalline silicon that, one way or another, are epitaxially connected to a singly crystalline silicon wafer. Most commonly, chemical vapor deposition (CVD) is used to epitaxially deposit silicon on a wafer consisting of a single crystalline region of silicon. The so deposited epitaxial and monocrystalline silicon film is thereafter processed into transistors or similar devices. However, a need has long existed to fabricate silicon transistors on glassy substrates or glassy intermediate layers.
In one approach, a layer of silicon is deposited over a glassy layer of silicon oxide. The deposited silicon is initially amorphous or polycrystalline, but annealing can make it form monocrystalline domains aligned with adjacent exposed crystalline regions of the underlying singly crystalline silicon substrate. One purpose for such a structure is to allow conduction only in the thin silicon surface layer, thereby preventing any parasitic interactions with the silicon substrate. In another approach, the silicon is deposited as a layer of high-quality amorphous silicon, and the amorphous silicon can be used as the active silicon region. This latter approach is used to form amorphous silicon transistors on glass substrates used for active-matrix flat panel displays.
However, the need remains to easily form singly crystalline silicon on disordered substrates or intermediate layers, such as silicon dioxide and silicate glasses.
Ramesh in U.S. Pat. No. 5,248,564 (the '564 patent) disclosed a powerful tool for the formation of crystallographically oriented metal oxides over a silicon dioxide (SiO.sub.2) film. He found that bismuth titanate (Bi.sub.4 Ti.sub.3 O.sub.12 or BTO) acts as an effective template for the overgrowth of crystallographically oriented metal oxides even when the BTO is grown on an amorphous SiO.sub.2 layer. As Ramesh has explained in U.S. Pat. No. 5,270,298 (the '298 patent), bismuth titanate is one of a class of layered perovskites having two approximately perpendicular and approximately equal a- and b-lattice parameters, a=0.541 nm and b=0.545 nm, and a perpendicular but substantially long c-lattice parameter, c=3.28 nm. Films of the layered perovskites in general and of bismuth titanate in particular have a strong tendency to grow with the c-axis perpendicular to the growth plane of the film, even when the underlying layer is amorphous SiO.sub.2. Thereafter, the metal oxide film is epitaxially deposited on the Bi.sub.4 Ti.sub.3 O.sub.12 template layer. As explained in the '564 patent, the metal oxide, which has a nominally cubic crystallographic structure, is believed to deposit as a layer forming a crystallographic mosaic. That is, one of the three equal lattice vectors axes is strongly oriented in the vertical (z-axis) direction but the other two lattice vectors are distributed in the plane of the film surface in separate two-dimensional crystallites. Ramesh's work follows prior work described by Inam et al. in U.S. Pat. No. 5,155,658 (the '658 patent) and by Ramesh et al. in U.S. Pat. No. 5,168,420 (the '420 patent). These last two patents disclose the growth of crystalline ferroelectrics, including Bi.sub.4 Ti.sub.3 O.sub.12, over crystalline perovskites. All four aforementioned patents are incorporated herein by reference.